1. Field of the Invention
This invention relates to an image forming apparatus having a sheet material transport device, and more particularly, to an image forming apparatus having a paper reversing mechanism and a reconveying path for duplex copying or printing and multiple copying or printing used for example, in a laser-beam printer, a copier or the like.
2. Description of the Related Art
The configuration of a conventinal image recording apparatus having a paper reversing mechanism will now be explained with reference to the drawings. FIG. 1 shows a paper conveyance path through a copier.
In FIG. 1, there are shown sheets of paper 1 and 1', cassettes 2 and 2', paper-feeding rollers 3 and 3', registration rollers 4 and 4', a photosensitive drum 5, a transfer charger 6, fixing rollers 7 and 7' and conveying rollers 8 and 8' within a fixing unit, a flapper 9, rollers 10 and 10' capable of rotating in forward and reverse directions, conveying rollers 11, 11' and 12, 12', a scanner 13, a turning mirror 14, and paper discharging rollers 15 and 15'.
FIG. 2 is an enlarged detail view of a portion of the paper conveyance path illustrated in FIG. 1.
The paper conveying operation when duplex printing is performed will now be explained.
In FIG. 1, a sheet of paper 1 fed from the cassette 2 by the paper-feeding roller 3 (or from the cassette 2' by the paper-feeding roller 3') is controlled by the registration rollers 4 and 4' so that the front end of the sheet of paper 1 coincides with the front end of an image formed on the photosensitive drum 5 using a known photographic process, and is then conveyed. The image is then transferred to the sheet of paper 1, and the transferred image is fixed by the fixing rollers 7 and 7'. The sheet of paper 1 on which the image is transferred is then guided to the flapper 9 by the conveying rollers 8 and 8'.
Referring to FIG. 2, in the case of performing printing on two surfaces of the sheet of paper 1, the flapper 9 is at a position 9a depicted by a solid line when the sheet of paper 1 first passes through the flapper 9.
The sheet of paper 1 proceeds in the direction A which is the paper conveying route indicated by solid line arrows because of the position of the flapper 9, and is further conveyed by the forward rotation of the rollers 10 and 10' capable of rotating in forward and reverse directions.
When a sensor (not illustrated) detects that the rear end of the sheet of paper 1 has passed a guide plate 16, the rollers 10 and 10' rotate in directions indicated by broken line arrows, and the sheet of paper 1 is conveyed in the reverse direction indicated by broken line arrows.
Referring again to FIG. 1, the sheet of paper 1 subjected to switchback by the rollers 10 and 10' is guided again to registration rollers 4 and 4' along the conveying path indicated by broken line arrows by the conveying rollers 11, 11' and 12, 12'. Printing is then performed on the second surface in the same way as when the first surface was printed. The second printed image on the sheet of paper 1 is also fixed via the fixing rollers 7 and 7', and the sheet of paper 1 with a fixed image on both sides is then guided to the conveying rollers 8 and 8'.
Referring again to FIG. 2, the flapper 9 is controlled so that it is switched to a position 9b indicated by a broken line when the sheet of paper 1 on which the second printing has been completed passes through it. The sheet of paper 1 conveyed by the conveying rollers 8 and 8' is directed along conveying path B by the flapper 9, and is discharged outside the apparatus (onto a paper-discharging tray 17) by the paper discharging rollers 15 and 15' which rotate only in one direction, as shown in FIG. 1.
Although the above-described apparatus has a configuration in which a part of a reversing sheet of paper is sent outside the apparatus, there are apparatuses in which a sheet of paper is reversed only within the apparatus. At that moment, a dedicated space for the reversal is needed.
However, in the conventional apparatus as described above, there are the disadvantages that, since a sheet of paper is subjected to switchback by rollers capable of rotating in forward and reverse directions in a paper reversing mechanism, a space for performing the switchback is newly needed, and hence the size of the apparatus becomes large and the cost becomes high.